Footlambert - AbsoluteAstronomy.com

A foot-lambert or footlambert (fL, sometimes fl or ft-L) is a unit of luminance

Luminance

Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square...

in U.S. customary units and some other unit systems. A foot-lambert equals 1/π candela

Candela

The candela is the SI base unit of luminous intensity; that is, power emitted by a light source in a particular direction, weighted by the luminosity function . A common candle emits light with a luminous intensity of roughly one candela...

per square foot

Square foot

The square foot is an imperial unit and U.S. customary unit of area, used mainly in the United States, Canada, United Kingdom, Hong Kong, Bangladesh, India, Pakistan and Afghanistan. It is defined as the area of a square with sides of 1 foot in length...

, or 3.426 candela per square meter (the corresponding SI unit). The foot-lambert is named after Johann Heinrich Lambert

Johann Heinrich Lambert

Johann Heinrich Lambert was a Swiss mathematician, physicist, philosopher and astronomer.Asteroid 187 Lamberta was named in his honour.-Biography:...

(1728–1777), a Swiss-German mathematician, physicist and astronomer. It is rarely used by electrical and lighting engineers, in favor of the candela per square foot or candela per square meter.

The luminance of a perfect Lambertian

Lambertian reflectance

If a surface exhibits Lambertian reflectance, light falling on it is scattered such that the apparent brightness of the surface to an observer is the same regardless of the observer's angle of view. More technically, the surface luminance is isotropic...

diffuse reflecting surface in foot-lamberts is equal to the incident illuminance

Illuminance

In photometry, illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of the intensity of the incident light, wavelength-weighted by the luminosity function to correlate with human brightness perception. Similarly, luminous emittance is the luminous flux per...

in foot-candle

Foot-candle

A foot-candle is a non-SI unit of illuminance or light intensity widely used in photography, film, television, conservation lighting, and the lighting industry...

s. For real diffuse reflectors, the ratio of luminance to illuminance in these units is roughly equal to the reflectance of the surface. Mathematically,

,
where

is the luminance, in foot-lamberts,

is the illuminance, in foot-candles, and

is the reflectivity, expressed as a fractional number (For example, a grey card with 18% reflectivity would have

.)

The foot-lambert is used in the motion picture industry for measuring the luminance of images on a projection screen

Projection screen

A projection screen is an installation consisting of a surface and a support structure used for displaying a projected image for the view of an audience. Projection screens may be permanently installed, as in a movie theater; painted on the wall; or semi-permanent or mobile, as in a conference room...

. The Society of Motion Picture and Television Engineers

Society of Motion Picture and Television Engineers

The Society of Motion Picture and Television Engineers The Society of Motion Picture and Television Engineers The Society of Motion Picture and Television Engineers (SMPTE , founded in 1916 as the Society of Motion Picture Engineers or SMPE, is an international professional association, based in...

(SMPTE) recommended, in SMPTE 196M, a screen luminance of 16 foot-lamberts for commercial movie theaters, "open-gate" with no film in the projector

Movie projector

A movie projector is an opto-mechanical device for displaying moving pictures by projecting them on a projection screen. Most of the optical and mechanical elements, except for the illumination and sound devices, are present in movie cameras.-Physiology:...

. (Typical base density of 0.05 yields peak white of about 14 fL.) The current revision of SMPTE 196M specifies 55 candela per square meter (nits

Nit (unit)

The candela per square metre is the derived SI unit of luminance. The unit is based on the candela, the SI unit of luminous intensity, and the square metre, the SI unit of area...

).

The foot-lambert is also used in the flight simulation industry to measure the highlight brightness of visual display systems. The minimum required highlight brightness varies based on the type and level of Flight Simulation Training Device (FSTD), but is generally 3–6 foot-lamberts for most devices qualified under Federal Aviation Administration

Federal Aviation Administration

The Federal Aviation Administration is the national aviation authority of the United States. An agency of the United States Department of Transportation, it has authority to regulate and oversee all aspects of civil aviation in the U.S...

(FAA) or Joint Aviation Authorities

Joint Aviation Authorities

The Joint Aviation Authorities, or JAA, was an associated body of the ECAC representing the civil aviation regulatory authorities of a number of European States who had agreed to co-operate in developing and implementing common safety regulatory standards and procedures...

(JAA) regulations.

See also